Hebrews 5:11
Concerning whom we have much to say, and that which is difficult to explain, because you have become dull of hearing.
2 Peter 3:16
as also in all his epistles speaking of these things, in which are some things hard to be understood, which the unlearned and unstable wrest as they also do the other Scriptures to their destruction.
Matthew 13:15
for the hearts of this people have become hard, and they hear imperfectly with their ears, and their eyes have they closed, lest they should see with their eyes, and hear with their ears, and understand with their hearts, and turn, and I should heal them.
Mark 8:17-18
And Jesus knowing it, said to them, Why do you reason because you have no bread? Do you not yet perceive nor understand? Is your heart hardened?
Mark 8:21
And he said to them, Do you not yet understand?
Luke 24:25
And he said to them, O foolish men, and of a mind slow to believe all that the prophets have spoken.
John 6:6
But this he said to try him; for he knew what he was about to do.
John 16:12
I have many things yet to say to you, but you cannot bear them now;
Acts 28:27
for the heart of this people has become hard, and they hear imperfectly with their ears, and they have shut their eyes, lest at any time they should see with their eyes, and hear with their ears, and understand with their hearts, and turn, and I should cure them.